Aircraft are known which are powered by a power plant comprising two propeller devices, each of which in turn comprises an engine, a propeller, and a gear transmission interposed between the engine and the propeller and having an output shaft to which the propeller is fitted. Each propeller device rotates, about a substantially horizontal axis, between a first work position in which the respective output shaft is vertical and the propellers permit vertical takeoff of the aircraft in the same way as a helicopter, and a second work position in which the output shaft extends substantially horizontally and the propellers permit forward flight of the aircraft.
In most applications, the two propeller devices are connected by a counter shaft, so that, in the event either of the engines breaks down, the other provides for rotating both propellers.
Though widely used, known power plants of the above type leave considerable room for improvement, on account of the considerable force required and the relatively long time taken to rotate between said two positions, mainly due to the weight of the components for rotation.
Solutions are known in which the engines remain stationary with respect to the aircraft. Such solutions, however, only provide for partly solving the problem, on account of most of the components of each propeller device still being rotated about the respective horizontal axis. Moreover, the movement of the propeller with respect to the relative engine makes the transmission extremely complex, expensive, and, in some case, heavier than movable engine transmissions.
It is an object of the present invention to provide a power plant for propeller aircraft, designed to provide a straightforward, low cost solution to the problems typically associated with the known state of the art.
According to the present invention, there is provided a power plant for propeller aircraft, comprising two propulsion devices; each propulsion device comprising a respective propeller, a respective engine for powering the propeller, and a gear transmission interposed between the engine and the propeller and comprising a power input shaft connected directly to said engine, and a power output shaft supporting said propeller and rotated with respect to said input shaft about a hinge axis substantially perpendicular to the output shaft; said gear transmission also comprising a single gear fitted to said output shaft and comprising two opposed sets of teeth, a pair of pinions, each meshing with a respective said set of teeth, and, for each said pinion, a single respective gear meshing directly with the gear of the other pinion; characterized in that one of said pinions extends coaxially with said hinge axis, and is connected to said input shaft by a single pair of gears.
The gears in said single pair of gears are preferably bevel gears.
This therefore provides for obtaining a power plant for propeller aircraft, in which the propellers rotate with respect to the aircraft, while the respective engines remain stationary, and in which the loads involved in effecting rotation are considerably reduced, thus permitting faster, more reliable rotation of the propellers with respect to the aircraft.
Moreover, the gear transmission of the power plant according to the invention is considerably reduced with respect to that of the known state of the art, thus reducing the weight of the power plant as a whole.